There are numerous factors affecting the growth of vegetable transplants including types of growing media, watering practices and fertilization programs.
Types of Growing Media
Growing media for vegetable transplants in greenhouses contain a variety of soilless ingredients such as peat moss, vermiculite, perlite, shredded coconut husks (coir), or composted materials plus starter nutrients and a wetting agent. Field soils are generally unsatisfactory for the production of plants in containers because soils do not provide the aeration, drainage and water holding capacity that are required. They also need to be pasteurized or fumigated to prevent the development of diseases and germination of weed seeds.
Premixed media is common in the greenhouse industry. Suppliers offer a diversity of mixes that are available prepacked (in bags, bales, super sacks) or in bulk. Growing media is designed to achieve high porosity and water retention while providing adequate aeration. Recipes are specially formulated for propagation, specific crops or general use. Soilless media purchased in bags do not have to be pasteurized or fumigated before use. Preventative applications of biological fungicides or fungicides may be necessary with vegetable transplants prone to damping-off. Growers can also obtain commercially available mixes with biological fungicides added to the mix. Those containing mycorrhizae, though increasingly common, may have been prepared many months in advance and may contain insignificant concentrations of living inoculum.
Compost-based mixes are also available commercially as a substitute for traditional soilless media, especially for organic production. See section below on organic vegetable transplant fertility.
Test your growing medium to adjust your fertilizer program and to manage the pH to prevent nutritional problems. Samples from soilless mixes are tested differently than samples from field soil. Unlike field soil tests that extract nutrients with weak acid solutions, soilless media and organic media with compost are mixed with distilled water at a standard dilution and then analyzed. There are three commonly used methods of testing soilless media using water as an extracting solution: saturated media extract (SME), 1:2 dilution method, and leachate Pour Thru. Most soil testing laboratories use the SME method. The values that represent each method of testing are different from each other. Likewise, values for specific nutrients are likely to differ with testing methods. Always use the interpretative data for the specific soil testing method used to avoid incorrect interpretation of the results. Most soil testing laboratories use the SME method. The 1:2 and Pour Thru are methods that can be used by growers on-site using portable soil testing meters. Since different soil testing laboratories may use different dilutions, it is not advisable to compare soil test results from one laboratory to those obtained from another. Use one laboratory for consistent results.
In addition to carrying out a complete soil test, growers should routinely check the electrical conductivity (EC) or soluble salts and pH of their growing media. These tests can be done on-site using portable testing meters, or samples can be sent to a university soil testing laboratory.
Taking a Sample: Take several samples at root depth from several containers and mix together in a clean container. Sampling several containers is important because a sample from one pot or flat could be an anomaly (values too high or too low) misrepresenting the crop as a whole. Sample about 2 hours after fertilizing or at least on the same day. If slow-release fertilizer pellets are present, carefully pick them out of the sample. If the pellets are left in, they can break during testing and this may result in an overestimation of fertility.
Be consistent in sampling procedures each time you sample. A lot of variability can be introduced to tests due to inconsistent sampling and this diminishes the value of testing especially if you are monitoring fertility levels over time.
Take about one to two cups of the medium and dry at room temperature. Place in a sandwich size zip-type bag and close it tightly. Label each sample on the outside of the bag with the sample bag or number. Be consistent in sampling procedures each time your sample.
pH: The term pH refers to a measurement of the hydrogen ion concentration (how acidic or basic a solution is). The pH can range from 0 (very acidic) to 14 (very basic). Growing medium pH drives the chemical reactions that determine whether nutrients are either available for root uptake (soluble) or unavailable for root uptake (insoluble). Major influences on the media pH include limestone in the growing media, irrigation water pH and alkalinity, and the acid/basic nature of fertilizer solution used. Smaller cells and plugs are subject to very rapid media pH change.
The optimum pH range for vegetable bedding plants grown in soilless media is 5.5-6.5.
Electrical Conductivity (EC) or Soluble Salts: Soluble salts are the total dissolved salts in the root substrate (medium) and are measured by electrical conductivity (EC). Most fertilizers (except urea) are salts and when placed in solution they conduct electricity. Measuring EC or soluble salts provides a general indication of nutrient deficiency or excess. A high EC reading generally results from too much fertilizer in relation to the plant’s needs, but inadequate watering and leaching or poor drainage may be other causes. Sometimes, high EC levels occur when root function is impaired by disease or physical damage. Always check the condition of the root system when sampling growing media for testing.
Water Quality and Alkalinity: The quality of water used for irrigation and mixing fertilizers should be tested each year for pH, alkalinity and electrical conductivity. A standard greenhouse water test includes pH, EC, alkalinity as well as macro and micronutrients (N, P, K, Ca, Mg, S,B, Cl, Cu, Fe, Mn, Mo and Zn) and sodium (Na). Testing should be done at least once per year. (This is a different test from the water test that is done for microbial quality). Water containing a large concentration of dissolved salts can cause high soluble salts damage.
Water alkalinity is a measure of the water's capacity to neutralize acids. An alkalinity test measures the level of bicarbonates, carbonates and hydroxides in water. Test results are generally expressed as ppm of calcium carbonate. Irrigation water tests should always include both pH and alkalinity. A pH test itself is not an indication of alkalinity. Water with high alkalinity (i.e., high levels of bicarbonates or carbonates) always has a pH value of 7 or above, but water with high pH does not always have high alkalinity. This is important because high alkalinity exerts the most significant effects on growing medium fertility and plant nutrition.
Water with high alkalinity (>150ppm CaCO3 of alkalinity) can result in iron deficiency chlorosis caused by increased root medium pH over time. This is usually influenced by the water source. Water with low alkalinity will have little ability to neutralize acidity. It is advisable to have your water tested prior to the spring growing season. High alkalinity can be reduced by injecting acid into the irrigation water. While most greenhouses use sulfuric acid, citric acid is approved for certified organic production. Calculate the amount of acidification required using the UMass Greenhouse Crops and Floriculture Program page: https://ag.umass.edu/greenhouse-floriculture/fact-sheets/adjusting-alkalinity-with-acids.
Fertilizers and Media pH
Most water-soluble fertilizers will change the potting media pH to some extent. Ammonium and urea-containing fertilizers lower media pH; nitrate fertilizers raise it. Potential acidity or basicity is printed on the fertilizer label based on pounds of calcium carbonate per ton of fertilizer. For example, if a 20-10-20 has a potential acidity of 429 pounds per ton, then the reaction produced by one ton of fertilizer will neutralize 429 pounds of calcium carbonate. If 15-0-15 has a potential basicity of 420 pounds per ton, then the reaction produced by one ton of the fertilizer will be equivalent to 420 pounds of calcium carbonate. Changes in pH of the media are caused by plant responses to the forms of nitrogen. Because fertilizers can be used to manipulate the pH of the growing media, most growers alternate fertilizers to balance the pH of the growing medium.
Fertilizer Injectors: In conventional greenhouses, nutrients are delivered using various water-soluble fertilizers through a fertilizer injector, through the use of controlled-release fertilizers, or using a combination of these two methods.
Fertilizer injectors are used in liquid feeding systems. These devices inject a small quantity of concentrated fertilizer solution (stock solution) into the irrigation line so that the water leaving the hose (dilute solution) supplies the proper concentration of fertilizer. When applied at every watering, this is known as “constant feed.” Rates of fertilization are often given in parts per million (ppm) of nitrogen, which is a way of expressing the fertilizer concentration. The amount of fertilizer to dissolve per gallon of water (stock solution) to make the appropriate concentrate for a specific injector setting needs to be determined. This information is listed on the bag of fertilizer. An injector setting of 1:100 indicates that 1 gallon of fertilizer concentrate delivers 100 gallons of final solution. It is not an indication that the injector is delivering 100 ppm of fertilizer solution.
Choosing Fertilizers: Factors to consider when choosing fertilizers include the ratio of ammonium to nitrate-N, trace element starter charge, content of calcium and magnesium, and potential acidity or basicity. There are many fertilizers available to use for vegetable bedding plant production. Commonly used fertilizers include 15-0-15 (Dark Weather Feed), 15-16-17 and 20-10-20, 15-5-15 ("Cal-Mag"), and 13-2-13.
Plug Production (15-5-15, 13-2-13) Both fertilizers combine high nitrate, low phosphate with extra calcium and magnesium, plus micronutrients. 15-5-15 is a little less basic (raises pH) than 13-2-13 and contains less nitrate and a little more phosphorus. Both are commonly used for plug production.
Peat-Lite Specials (15-16-17, 20-10-20, 21-5-20). These fertilizers are among the most popular for routine fertilization of vegetable bedding plants. All are high (>50%) nitrate fertilizers. However, these fertilizers also have elevated trace element levels which may raise iron (Fe) and manganese (Mn) to toxic levels at low pH. All are acid-forming fertilizers, but 20-10-20 has the greater potential acidity.
General Purpose (17-5-17, 17-3-17, 17-4-17). All are high (>50%) nitrate fertilizers with calcium, magnesium and other minor elements. All produce a nearly neutral reaction.
General Purpose (20-20-20). Growers who use this fertilizer with soilless media risk ammonium toxicity because the nitrogen in this fertilizer is 75% ammonium and urea. Some growers who use media containing soil do not appear to have problems. If 20-20-20 is used, the soilless growing medium should be tested frequently for ammonium. 20-20-20 supplies trace elements and has the greatest potential acidity of fertilizers commonly used in New England greenhouses. Tomato, eggplant and pepper plants are especially sensitive to ammonium, reducing plant growth and causing yellowing of the foliage.
Low Phosphorus (P) Fertilizers (20-0-20, 20-1-20, 15-0-15). These fertilizers can be tried as an alternative to chemical growth regulators for vegetable transplants. This technique of growth control is sometimes called "phosphorus starvation." It is generally believed that more P than necessary is being applied to greenhouse crops. Too much P may cause plants to stretch and P is a ground water pollutant. Unfortunately, in terms of height control, these fertilizers may be of no benefit if they are applied to a growth medium containing superphosphate or a high starter charge of P. Also, there is a risk of P deficiency if the fertilizers are used continuously with low P growth media. The low P fertilizers are quite different in many ways. 15-0-15 and 20-0-20 supply Calcium (Ca). 15-0-15 is a basic (raises pH) fertilizer containing about 95% nitrate and 20-0-20 is a neutral fertilizer and is 50% nitrate. 20-1-20 is an acidic fertilizer and it does not supply Ca, but it is about 70% nitrate.
Calcium Nitrate and Potassium Nitrate (15-0-15). High nitrate, high calcium fertilizer. Some growers alternate its use with the Peat-Lite Specials on a 2-3 week basis to supply Ca and to counter the acidic effect of the Peat-Lite fertilizers. If water-soluble NPK fertilizer is not applied at least once every 10-14 days, superphosphate must be incorporated into the growing medium.
Guidelines for Rates and Frequency of Fertilizer
Small, slow-growing plants should receive lower rates or less frequent application until they are well-established. Care should be taken not to over-fertilize vegetable transplants to avoid overgrown plants. Young seedlings are especially vulnerable to injury from high soluble salts.
While plants are in the plug or seedling stage, use a complete water-soluble fertilizer at 50-100 ppm N every time plants are watered and use clear water (no fertilizer) every third watering. Use the lower rate (50 ppm) early and the higher rate (100 ppm) later if the seedlings are to be held in the flat or tray three or more weeks before transplanting. Shortly after transplanting, as plants approach rapid growth, increase the rate to 200 ppm N at every watering or 300 ppm N once every 7 days, watering with clear water 2-3 times between each fertilization.
Fertilizer Solution Volume: The volume of fertilizer solution applied has a dramatic effect on the growth of the vegetable transplants. As the volume of water-soluble fertilizer increases, the quantity of nutrients delivered to the plant also increases resulting in an increase in height, stem diameter and plant weight. Doubling the volume applied also doubles the amount of each nutrient potentially available to the plant.
Plant Growth Rate and Environmental Conditions. In general, nutrient requirements of vegetable transplants are greatest during periods of rapid growth. Too much fertilizer during slow growth periods may lead to high soluble salts; failure to provide enough fertilizer during periods of rapid growth will lead to nutrient deficiency.
Early in production, serious nutritional problems include high soluble salts, trace element toxicities, and ammonium toxicity. Later in production, particularly in cell packs, plants may develop nitrogen deficiency symptoms as the earliest indication of insufficient fertility levels.
Soluble Salts. Injury to vegetable transplants from excess salts seems to be most common shortly after transplanting. The major sources of high salts are excessive fertilizer from liquid feeds or media from sources that contain high concentrations of salts. Excessive liquid fertilizer can be applied if your fertilizer injector is not calibrated or working properly. High salts can also come from compost based growing media. The quality of the compost depends upon what it was made from and how well it was finished (how long it was allowed to mature). Seedlings are much less tolerant to excess salts than established, rapidly growing plants. Some soilless mixes may contain enough "starter charge" to cause excess salts problems in the first few weeks after transplanting, particularly when a water-soluble fertilizer is also applied. Excessive drying, poor drainage, and uneven watering are factors that can aggravate this problem. Check roots of plants often and conduct regular soil tests to identify and prevent problems. It is difficult to diagnose a soluble salts problem by symptoms alone. Often nutrient deficiencies and root diseases cause the same symptoms, along with inadequate watering. A soil media test is advisable if you suspect salt injury.
Trace Element Toxicities. Iron (Fe) and/or manganese (Mn) can be accumulated to toxic levels by tomato plants. Symptoms appear as numerous small dark spots and mottling of the foliage. The potential sources of excess Fe and Mn are: trace element fertilizers in the mix, water-soluble fertilizers with elevated trace elements levels, and sometimes irrigation water. Low growth medium pH aggravates the problem by increasing Fe and Mn availability. Toxicity can be avoided by keeping the pH in the range of 5.8-6.0 for susceptible crops and by the use of fertilizers with lower trace element levels.
Ammonium Toxicity. During cool growing conditions, (less than 60°F), wet growing media and low pH, nitrifying bacteria are suppressed so that ammonium may build up to toxic levels. Tomato, eggplant, and pepper transplants are particularly sensitive to high levels of ammonium, but other vegetable transplants can also be damaged. Symptoms of ammonium toxicity include yellowing or chlorosis between the veins, and scattered necrotic spots. Plants may be stunted. At first, young leaves are affected, but later, older leaves show symptoms. Root tips are also damaged.
Nitrogen and Phosphorus in Transplant Production. Nitrogen concentration in the greenhouse fertilizer program has a greater effect on the growth of transplants than either phosphorus or potassium. Raising the level of nitrogen results in taller transplants with thicker stem diameters and heavier plant weights, but applying too much nitrogen may result in soft, poor quality transplants. These lush transplants may also be more prone to phloem-feeding insects such as aphids, whiteflies and to foliar blights. Phosphorus has a limited effect on the growth of transplants when compared to nitrogen, but should be included as part of a complete fertilizer. Increasing the phosphorus concentration results in a moderate increase in transplant height, stem diameter, and shoot fresh and dry weight. If phosphorus is restricted to the point at which the plants show extreme phosphorus deficiency (purple leaves and stems, stunted plants), field performance will be reduced.
Organic Vegetable Transplant Production
Growing Media and Nutrition
Types of growing media
Conventional growing media containing synthetic ingredients (wetting agent, starter chemical fertilizer) cannot be used in organic production of vegetable transplants. However, acceptable growing media can be created from a wide variety of approved materials. These blends for organic production may be purchased off-the-shelf, custom-blended by manufacturers, or produced on-the-farm.
Purchasing a commercially prepared mix for organic production is the easiest way to get started and most growers choose this option to ensure consistency and reduce the risk of soilborne diseases. Common components such as peat moss, perlite, vermiculite, and coconut coir are acceptable for organic certification. Commercial mixes for organic production may contain a "starter charge" of organic fertilizer or no starter fertilizer at all. Check with your organic certifier to make sure your mix complies with standards. More information on growing media for organic production can be found in the ATTRA publication, " Potting Mixes for Certifed Organic Production".
Use of Compost based growing mixes
Compost is a renewable resource that can be purchased locally or made from locally sourced materials and may be many organic growers' preferred growing mix base. Other advantages of adding compost to growing mixes is that it has good cation exchange properties, can function as a wetting agent in peat-based mixes, and its microbial activity may help suppress diseases. However, poorly made unfinished compost can be high in soluble salts, contain weed seeds, may emit ammonia, and may be a source of pathogens such as Pythium, Phytophthora, and Rhizoctonia.
There are many challenges to using compost in growing mixes because its chemical and physical properaties vary from batch to batch. Because each batch of compost is different, it is critical to test the media before use. (See previous section on media testing). In addition to sending samples to a laboratory for testing, you can also do in-house bio-assays, seeding some quick growing oats, onions, beans or radish seedlings. Plant these quick-growing seeds several weeks before you plan on using the mix to see how they grow and perform. You can also plant these seeds in a soilless mix to see how these seedlings compare to seedlings grown in compost.
Before purchasing a compost-based mix, ask your vendor for more specific information on its characteristics. Composts with the US Compost Seal of Approval must meet specific standards. For use in organic production, composts must meet organic standards and be OMRI approved. Compost is rarely used by itself as a potting medium. Mixes may contain from 30 to 50% compost by volume. In research at the University of Rhode Island, mixes with 36% compost combined with peat, perlite, and vermiculate yielded a mix with desirable bulk density.
Nutritional Problems Some potenial nutrient issues with compost-based mixes include high excess salts, high ammonium levels, high levels of sodium, and low nutrient levels such as potassium. The quality of the compost depends upon what it was made from and how well it was 'finished" ( how long it was allowed to mature). Finished composts may contain 5.0 mS/cm soluble salts or more depending upon the feedstocks used. However, most vegetable seedlings only tolerate a soluble salt level of 1.0 mS/cm.
When using compost-based growing mixes, the aerobic bacteria that are needed to convert ammonium nitrogen to nitrate nirogen are dependent upon environmental conditions. The speed of this reaction depends upon both the temperature and microbial activity. During cool growing conditions, (less than 60ºF), with wet growing media and low pH, nitrifying bacteria are suppressed so ammonium may build up to toxic levels. Compost that is high in nitrogen sources, such as poultry litter or food wastes, can be high in ammonium and other nutrients. Conversely, if there are low levels of nutrients, transplants can be stunted and not perform well in the field.
Physical Properties The physical properties of a compost-based mix can also differ from conventional greenhouse potting mixes. Compost may contain excessive amounts of fine-sized particles, so that the mixes hold moisture longer than desired. Compost has a higher bulk density that may be 3-4 times the bulk density of peat. (Bulk density is the weight of a given volume of material). High bulk density is an indicator of low porosity and media compaction that may cause restrictions to root growth, and poor movement of air and water through the media. In greenhouse mixes, a low bulk density is desirable. As organic matter decomposes, it tends to have a small particle size with poor drainage, and low porosity that can adversely affect root growth. Because composts are microbially active, they break down organic matter in the mix, resulting in compacted media. This compacted media adversely affects root growth. Compost-based mixes with too many fine particles make it difficult to manage moisture levels in the growing media. When you water, it may not penetrate the growing media and the media stays wet too long. Algal scum can then develop on the surface of the growing media.
Use of Supplemental Organic Fertilizers
Supplementing pre-plant fertilizers or compost with liquid organic fertilizers is generally required to provide adequate nutrition. Fish fertilizers, made from waste products of the ocean fish processing industry, are thick, heavy liquids which are difficult to use with fertilizer injectors because the concentrate consists of very fine particles in suspension. Dilute solutions develop a strong odor in storage. Because of this, fertilization may need to be less frequent. Application rate will depend on frequency. However, excessive fish fertilizer builds up a nutrient rich scum on the surface of media, leading to algal growth and shore fly proliferation.
Different fish fertilizers supply plant nutrients at varying levels of availability. Some may be stabilized with phosphoric acid, resulting in a high concentration of readily available phosphorus. Others contain liquid seaweed resulting in a small addition of potassium. Most fish fertilizers contain ammonium nitrogen which as discussed previously, can be a problem for sensitive transplants such as tomatoes, peppers and eggplants. In New England, the Neptune’s Harvest Brand is the most commonly available fish fertilizer and it is OMRI-approved for organic production.
Some growers use Nature's Source Professional Plant Food 10-4-3, a liquid, “organically-based” fertilizer. The organic portion is oilseed extract. Most of the nutrients, however, are derived from inorganic salts and for this reason it cannot be used for certified organic production.
Several liquid fertilizers used for organic production are derived from plant extracts. The best known of these has been Nature's Source Organic Plant Food 3-1-1, in which the nutrients are derived from “oilseed extract”. The container has dilution rates expressed in familiar terms for greenhouse growers and has been recommended based on trials at the University of Massachusetts.
Several other liquid organic products are available, such as Biolink 3-3-3 (also an oilseed extract), Converted Organics 3-2-1 (a byproduct of grain fermentation), 1-1-1 Liquid Compost Concentrate and Verdanta PL-2, 2-0-6 (a liquid made from fermented sugar cane and sugar beet molasses). Verdanta would be used as a supplement to use in combination with other fertilizers used for organic production that are low in N or K.
Verdanta EcoVita 7-5-10 (granular) is composed of bone meal, soybean meal, cocoa shell meal, feather meal and fermented sugar cane and sugar beet molasses.
Mixing and application. The fish fertilizers and plant extract fertilizers are sold as concentrates and they must be diluted in water to be safe for plants. Nature’s Source, Bombardier, and Espartan have a pleasant “beer-like” aroma as concentrates, but within 7 days of being mixed with water they “spoil” and develop unpleasant odors. The nutrient value of spoiled fertilizer is unknown and the colonies of bacteria that develop may plug irrigation lines, so diluted fertilizer solution should be used as soon as possible after mixing.
Fish fertilizer has the thickest and least consistent solution and should be agitated before mixing with water. Bombardier and Espartan concentrates are “syrupy” but mix well with water. Nature’s Source is the thinnest concentrate and it mixes well with water and can pass through fertilizer injectors.
Sustane 8-4-4 and EcoVita are granular fertilizers mixed with the growing medium before planting. These are the easiest organic nutrient sources to use in combination with the liquid types.
Fertilizer analysis. Some fertilizers used for organic production supply only one or two of the NPK elements; an example is Bombardier, which is 8-0-0. A grower using Bombardier would have to use other fertilizer(s) to supply P and K. One possibility would be Sustane with an 8-4-4 analysis or some other complete NPK granular organic fertilizer.
Nutrient disorders. Plants may develop an overall light green or yellowed color caused by a general nutrient deficiency or more likely, N deficiency. For example, if Sustane is used alone, the symptoms might occur about 45 days after planting, the end of its release time. This can be prevented by applying an organic liquid fertilizer supplement about 30 days after planting.
Interveinal chlorosis sometimes occurs about halfway through cropping time if plants are only fertilized with some liquid organic fertilizer starting at planting. This chlorosis is most likely caused by an accumulation of too much ammonium-nitrogen in the plant, so-called “ammonium toxicity.” Most greenhouse crops do best with a combination of ammonium and nitrate nitrogen. Unfortunately, fertilizers used for organic production generally don’t contain nitrate-nitrogen. The best approach is to rely on Sustane as the sole source of nutrients for the first month after planting and then start applying Nature’s Source or another liquid organic fertilizer.
Use fertilizers for organic production with caution, on plants you know have exacting nutrient requirements or those prone to foliar chlorosis. Fertilizers should always be tried first on a small number of plants.
Organic Transplant Fertilizer Resources
Cox D. 2016 Plant Response to Nature's Source and EcoVita Organic Fertilizers vs Plantex Chemical Fertilizer. July-Aug. Floral Notes 29(1).
Cox D. 2014. Organic Fertilizers - Thoughts on Using Liquid Organic Fertilizers for Greenhouse Plants. Sept.-Oct. Floral Notes 27(2)
Grubinger, V. 2012. Potting Mixes for Organic Growers. https://www.uvm.edu/vtvegandberry/factsheets/OrganicPottingMixes.pdf
Mattson N. 2014. Substrates and Fertilizers for Organic Vegetable Transplant Production. Cornell Greenhouse Horticulture, Cornell University. http://blogs.cornell.edu/greenhousehorticulture/crops-culture/substrates-and-fertilizers-for-organic-vegetable-transplant-production/
Organic Greenhouse Vegetable Production, Potting Mixes for Certified Organic Production, Organic Greenhouse Tomato Production, Plug and Transplant Production for Organic Systems, ATTRA - National Sustainable Agriculture Information Service. https://attra.ncat.org/wp-content/uploads/2022/11/plug-and-transplant-production.pdf
Organic Potting Mix Basics: eXtension https://eorganic.org/node/3442
Radin, A. 2020. Grow Your Own Seedlings? Let's talk about the media. URI Cooperative Extension. The Week in Vegetables. February 8, 2020. https://web.uri.edu/coopext/files/2-8-20-TWIV.pdf